In the design of integrated circuit systems, square wave logic level signals are used throughout the systems to communicate information and to trigger events. However, the rapid rise and fall times of square wave signals produce electromagnetic radiation. The faster the rise or fall transition rates, the higher the electromagnetic radiation. Likewise, the higher the current or voltages being changed, the higher the electromagnetic radiation. In many applications, this electromagnetic radiation can cause interference with other electrical systems. This phenomenon is referred to as electromagnetic interference (EMI).
In electronic applications with switching circuitry it is necessary to minimize the electromagnetic radiation which causes electromagnetic interference in other electronic devices. One method of lowering EMI is limiting the rise and fall transition times of signals generated by switches in the circuitry. This is especially true of switches which drive wires or cables where radiation cannot be easily suppressed with shielding techniques. The switching circuitry must control the rise and fall transition times of the switched signals such that they are slow enough to prevent unacceptable EMI radiation but not so slow as to degrade the performance of the system.
Switching may be accomplished by various kinds of driver circuits in modern electronic devices. A "high side" drive circuit drives, or switches, a positive supply to a load; this kind of driver is not required to sink significant currents when driving low. A "low side" driver, drives, or switches, a negative supply to a load. A "totem pole" driver is a hybrid of the low and high side drivers in which the driver alternately switches a load to a positive supply and a negative supply.
Prior driver circuits suffered from numerous disadvantages. One disadvantage in prior driver circuits is that they do not control the transition rates of the switched voltage or current to the tight tolerances required for many modern electronic applications. Another disadvantage of prior driver circuits is that the transition rate often varies depending upon the size of the voltage or current switched, thereby causing timing variations in the switch. A further disadvantage is that prior driver circuits do not adequately control the rise and fall transition rates within a tight tolerance of maximum and minimum times that is required while switching a voltage or current that may vary over a wide range.